Paleodynamics: Solar System Formation and the Early Environment of the Sun

Abstract

Our present paradigm of the formation of stars and planetary systems suggests that the early Solar System may have existed in much denser environments than the galactic disk, e.g., the cores of giant molecular clouds and open stellar clusters. Some constraints on a hypothetical primordial environment can be inferred from the present dynamical state of the Sun and planets. Tremaine (1991, Icarus 89, 85-92) has proposed that the large obliquities of the outer planets may have arisen from twisting of the ecliptic plane during the initial collapse of the protosolar cloud. I find that the current solar obliquity could only have been produced in the very dense environment of the initial collapse of the protoplanetary cloud: Mean post collapse densities would have a neglible effect on Solar System dynamics. The inclinations of the orbits of Uranus and Neptune, sensitive to perturbations on time scales of less than 1 myr, place a constraint on the number density of stars in the solar neighborhood. I place a 3σ limit on the product of the stellar density and residence time of 3 × 104 M ⊙ pc -3 Myr, still consistent with the average densities and lifetimes of observed open clusters. I examine the possible effect of the tidal field on planet formation. Tidal torquing can suppress runaway accretion of a massive body by maintaining a high velocity dispersion. I derive a limiting semimajor axis of ∼ 50 AU, but find that the sensitivity to parameters does not permit the observed extent of the planetary system to be used to infer limits on the tidal field during the epoch of planet formation. Stronger tides and a higher stellar density on a time scale of 10 8 year will affect comet cloud formation. I conclude that a transient comet cloud of a few M ⊕ could have formed at a few thousand atomic-units but this would have been rapidly destroyed by stellar encounters over 10 8 year. Uranus- and Neptune-crossing planetesimals could have been scattered into a belt about 200 AU from the Sun. Although the dynamical lifetimes of these objects are comparable to the Solar System, a time-scale comparison suggests that little mass could have reached such a cloud before the end of the cluster epoch.